Conventionally, a three way catalyst (catalytic unit for exhaust gas purification) is provided to an exhaust passage of an internal combustion engine in order to purify an emission discharged from the engine. As is well known, the three way catalyst has an “oxygen storage function” to store oxygen flowing into the three way catalyst, and discharge the stored oxygen. The three way catalyst is hereinafter simply referred to as a “catalyst.”
One of the conventional air-fuel ratio control apparatuses (hereinafter, referred to as a “conventional apparatus”) includes a downstream-side air-fuel ratio sensor disposed in the exhaust passage of the engine and downstream of the catalyst. The conventional apparatus determines a “base fuel injection amount to have an air-fuel ratio of a mixture supplied to the engine coincide with the stoichiometric air-fuel ratio” based on an amount of air introduced into cylinders, and corrects the base fuel injection amount based on at least an output value of the downstream-side air-fuel ratio sensor.
Hereinafter, an exhaust gas flowing into the catalyst is referred to as a “catalyst inflow gas”, and an exhaust gas flowing out from the catalyst is referred to as a “catalyst outflow gas.” Further, an air-fuel ratio which is smaller than the stoichiometric air-fuel ratio is referred to as a “rich air-fuel ratio”, and an air-fuel ratio which is larger than the stoichiometric air-fuel ratio is referred to as a “lean air-fuel ratio.” The air-fuel ratio of the mixture supplied to the engine is referred to as an “air-fuel ratio of the engine.”
The downstream-side air-fuel ratio sensor used for the conventional apparatus is typically a concentration-cell-type oxygen sensor utilizing a stabilized zirconia. As shown by a curve line C1 in FIG. 3, the output value Voxs of the downstream-side air-fuel ratio sensor coincides with a value close to a maximum output value Max when a state continues in which an air-fuel ratio of the catalyst outflow gas is smaller than the stoichiometric air-fuel ratio. The output value Voxs of the downstream-side air-fuel ratio sensor coincides with a value close to a minimum output value Min when a state continues in which the air-fuel ratio of the catalyst outflow gas is larger than the stoichiometric air-fuel ratio. Further, the output value Voxs of the downstream-side air-fuel ratio sensor rapidly changes from the value close to the maximum output value Max to the value close to the minimum output value Min, when the air-fuel ratio of the catalyst outflow gas changes from the rich air-fuel ratio to the lean air-fuel ratio. The output value Voxs of the downstream-side air-fuel ratio sensor rapidly changes from the value close to the minimum output value Min to the value close to the maximum output value Max, when the air-fuel ratio of the catalyst outflow gas changes from the lean air-fuel ratio to the rich air-fuel ratio.
In this manner, the output value Voxs becomes the value close to the minimum output value Min, when the air-fuel ratio of the catalyst outflow gas is the lean air-fuel ratio, and thus, the catalyst outflow gas includes an excessive amount of oxygen. The output value Voxs becomes the value close to the maximum output value Max, when the air-fuel ratio of the catalyst outflow gas is the rich air-fuel ratio, and thus, the catalyst outflow gas does not include an excessive amount of oxygen. Accordingly, it is inferred that the air-fuel ratio of the catalyst outflow gas is equal to the stoichiometric air-fuel ratio, when the output value Voxs coincides with a mid value Mid (i.e., the mid value Vmid=(Max+Min)/2) which is a middle value of the maximum output value Max and the minimum output value Min.”
The conventional apparatus calculates, based on a proportional-integral control (PI control), an air-fuel ratio feedback-control-amount, in such a manner that the output value Voxs of the downstream-side air-fuel ratio sensor becomes equal to a “target value VREF which is set to (at) a value (i.e., the mid value Vmid) corresponding to the stoichiometric air-fuel ratio.” The air-fuel ratio feedback-control-amount is also referred to as a “sub feedback amount”, for convenience. The conventional apparatus performs the feedback control of the air-fuel ratio of the mixture supplied to the engine by correcting the base fuel injection amount with the sub feedback control amount (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-171982).
FIG. 28 is a timing-chart showing an aspect of the air-fuel ratio feedback control performed by such a conventional apparatus. The conventional apparatus maintains the target value VREF at a constant value (reference value Vf close to the mid value Vmid), and determines whether the air-fuel ratio of the catalyst outflow gas is the rich air-fuel ratio or the lean air-fuel ratio. In other words, the conventional apparatus determines, based on the “output value Voxs and reference value Vf”, an “air-fuel ratio of the engine (required air-fuel ratio) which is required to purify the exhaust gas more efficiently with the catalyst.”
More specifically, when the output value Voxs is larger than the reference value Vf (e.g., time t1 to time t2, time t3 to time t4, and time t5 to time t6), the conventional apparatus determines that the air-fuel ratio of the catalyst outflow gas is the rich air-fuel ratio, and thus the requested air-fuel ratio is the lean air-fuel ratio (that is, the lean request has been occurring). When the lean request is occurring, the conventional apparatus controls/adjusts the air-fuel ratio of the engine to (at) the lean air-fuel ratio.
Consequently, the air-fuel ratio of the catalyst outflow gas changes to the lean air-fuel ratio, and thus, the output value Voxs decreases and becomes smaller than the reference value Vf. When the output value Voxs is smaller than the reference value Vf (e.g., time t2 to time t3, and time t4 to time t5), the conventional apparatus determines that the air-fuel ratio of the catalyst outflow gas is the lean air-fuel ratio, and thus the requested air-fuel ratio is the rich air-fuel ratio (that is, the rich request has been occurring). When the rich request is occurring, the conventional apparatus controls/adjusts the air-fuel ratio of the engine to (at) the rich air-fuel ratio. Consequently, the air-fuel ratio of the catalyst outflow gas changes to the rich air-fuel ratio, and thus, the output value Voxs increases and becomes larger than the reference value Vf.